The present invention relates to a hydraulic control system for a continuously variable V-belt transmission.
A line pressure used in the continuously variable V-belt transmission should preferrably be maintained at a minimum value required. If the line pressure is higher than this minimum value, the V-belt will be subject to a tension greater than necessary, shortening its life span and reducing its power transmission effeciency. Further, an oil pump has to discharge oil pressure higher than necessary, resulting in a loss of the pump.
Co-pending U.S. patent application Ser. No. 362,488, filed Mar. 26, 1982 discloses a hydraulic control system for a continuously variable V-belt transmission wherein a drive pulley has a larger pressure acting area therein than a pressure acting area in a driven pulley. A line pressure regulator valve used in this hydraulic control system generates a line pressure which varies linearly in proportion to an engine torque and also to a reduction ratio between the both pulleys. This line pressure approximates to the minimum value required over most of the operating range. But, it is still higher than necessary in certain operating range. Referring to FIG. 3, this will be further described. As shown by the fully drawn curves shown in FIG. 3, the minimum oil pressure P1 required by the drive pulley cylinder chamber is higher than the minimum oil pressure P2 required by the driven pulley cylinder chamber at reduction ratios larger than a certain reduction ratio near one and the former is lower than the latter at reduction ratios smaller than the certain reduction ratio. The line pressure according to the above co-pending U.S. application varies linearly as shown by the phantom line for the reduction ratio and thus becomes higher than necessary at reduction ratios near one. In order to provide the required minimum oil pressure over the whole reduction ratios, the line pressure must vary non-linearly as shown by the broken lines in FIG. 3.